1. Field of the Invention
The present invention relates to a two stage method for the desulfurization and oxidation of carbonaceous fuels and is particulary suitable for use in boiler retrofit applications whereby the combustible gas obtained in a first stage partial oxidation unit may be utilized as a primary fuel in the second stage oxidation unit, which preferably comprises a boiler combustion unit. Sulfur contained in the original carbonaceous fuel is removed for disposal as sulfur bearing slag granules.
2. Description of the Prior Art
The use of carbonaceous fuels, both solid and liquid, is of course well known in the prior art as an energy source. However, in recent years users of carbonaceous fuels throughout the world have become more and more concerned with the adverse effects on our environment, and particularly air quality as a result of burning carbonaceous fuels having a high sulfur content. Of particular concern in the prior art have been various methods and devices for "capturing" and removing sulfur dioxide gas generated upon combustion of such fuels. This problem has become relatively more extreme in recent years because of both the rising costs and relative scarcity of low sulfur solid and liquid carbonaceous fuels.
With particular regard to high sulfur carbonaceous fuels such as coal, the prior art literature is replete with numerous means for gasifying the coal to obtain a hot gaseous fuel while at the same time removing the sulfur therefrom. U.S. Pat. No. 4,062,657 to Knuppel discloses a method and apparatus for desulfurizing in the gasification of coal. This patent teaches the use of molten iron as a heat transfer media and chemical reactant for removal of sulfur during gasification of the coal. The patent further teaches that coal, lime and oxygen are introduced into the molten iron bath through bottom mounted tuyeres. The overall effect of this process is that the sulfur, as calcium sulfide, ends up in a slag layer which floats on the molten iron that flows to a separate chamber where the slag is desulfurized through reaction with oxygen to obtain calcium oxide and sulfur dioxide.
U.S. Pat. No. 2,830,883 to Eastman also discloses a process for gasification of solid carbonaceous fuels including sulfur. This process calls for the introduction of coal, lime, water and oxygen vertically downward into a reactor vessel. The product gas exits through the side of the vessel and is immediately quenched with water. The slag drops into a water bath in the bottom of the vessel where it is transferred to a clarifier for settling. In accord with the disclosure of that patent the reactor is designed for operating temperatures above 2,000.degree. F. and operating pressures of 100 psig or greater.
Other prior patents also teach the use of alkalis to remove sulfur as either hydrogen sulfide or sulfur dioxide in situ in a gasifier or fluid bed combustor, or from hot gas exiting a gasifier. These patents are as follows:
______________________________________ Inventor U.S. Pat. No. ______________________________________ Squires 3,481,834 Sass 3,736,233 Gasior 3,970,434 Van Slyke 3,977,844 Collin 4,026,679 Harris 4,092,128 Wormser 4,135,885 Kimura 4,155,990. ______________________________________
Accordingly, it is clear that it is known to remove sulfur in a gasification process based upon the reactivity of a basic slag to react with hydrogen sulfide. The U.S. Bureau of Mines reported this phenomenon during their experimental pulverized coal gasification pilot plant work in the early 1950's. Slag bath reactors such as the Rummel gasifier developed in Germany and incorporating feed nozzles that are above molten slag have been used for such gasification. However, the gasifiers required large water wall boiler sections to provide for adequate carbon conversion and slag quenching before the hot gases exited the gasifier proper. This was necessary for these gasifiers were not close coupled to a boiler. Of course, other alternatives for the removal of sulfur compounds from carbonaceous fuels and the exhaust of their combustion are also known in the prior art.
Chemical desulfurization of coal may be accomplished, and this results in coal of very fine particle size and an associated degree of carbon loss. If desulfurization is accomplished at a mine mouth, transportation by any means other than coal slurry is extremely difficult due to the resultant fine coal particle sizes. If desulfurization is accomplished at the point of use, solids disposal can present a problem. Technology clearly exists for chemical desulfurization of coal, but the method is fairly expensive and is not known to be in use in a commercial plant today.
Coal liquefaction is another alternative, but is expensive and considering economics, must be accomplished near the mine mouth. The necessary technology is quite sophisticated, and the resulting product is relatively expensive.
Conventional coal gasification followed by conventional hydrogen sulfide removal, from an economic viewpoint, simply does not appear to be a viable application for producing a boiler fuel. Only if the gas from the gasifier were already low in hydrogen sulfide and the gas could be kept above its dew point, would such conventional gasification appear to be a working alternative. Obviously, though, the use of carbon, high in sulfur content, would not be indicated; the necessary hydrogen sulfide removal feature is not present.
Finally, coal combustion followed by sulfur dioxide removal is commercially proved and operable, although the reliability of such a system is still sometimes questionable. A penalty on efficiency is paid due to flue gas pressure drop through the sulfur dioxide scrubber. Booster fans and reheating of flue gas after scrubbing results in overall efficiency losses of 1-2%, or loss of available power to sell of 3-6%. Accordingly, such systems are relatively costly, and in many cases a sludge is produced which is quite difficult to dispose of.
It is therefore apparent that there is a great need in the art for an economical, yet effective, method for desulfurizing and oxidizing carbonaceous fuels. Such a method would permit the utilization of high sulfur fuels at low capital expense and operating costs. It would furthermore be desirable if such a method would be suitable for producing a gaseous fuel which might be directly fed to existing coal and oil fired boilers as well as for use in new installations. Preferably, 55-90 wt. % of the sulfur content of the carbonaceous fuel should be removed, any auxillary power requirements associated with desulfurization and oxidation should be minimized, and the sulfur-containing waste material should be innocuous with regard to environmental concerns associated with solids disposal.